Chemically modified hemoglobins present an important approach to providing alternatives to transfusion of donated blood. Chemical modifications, chiefly by treatment with cross-linking agents, prevent tetrameric hemoglobin from dissociating into dimers which leads to rapid renal elimination. These dimers also may cause kidney damage. Cross-linked hemoglobin has been previously described for alpha, alpha cross-linkages in U.S. Pat. Nos. 4,598,064 and 4,600,531 (Walder) where the alpha chains are cross-linked by use of the cross-linking agent bis (3,5-dibromosalicyl) fumarate.
The use of cross-linking agents in preparing modified hemoglobins for therapeutic administration requires that the cross-linking agent and its precursors be of pharmaceutical purity. This means that a method of synthesizing the cross-linking agent or its precursors must not yield by-products that co-purify with the desired compound thereby risking contamination of the final hemoglobin. By pharmaceutical purity, it is meant that the substance be substantially free of any toxic compound. In general, a cross-linking agent of choice is formed in a reaction generalized in the following equation: ##STR1## wherein R is a lower alkyl or alkylene of from 1 to about 5 carbon atoms; Y is hydrogen, chloro-, bromo-, iodo-, nitro-, cyano- or trifluoromethyl-, and n is 1 or 2. Particularly efficacious in this reaction is 3,5-dibromosalicylic acid.
3,5-dibromosalicylic acid is also an important precursor in the synthesis of 3,5-dibromoacetylsalicylic acid. This compound is the dibrominated derivative of aspirin, and like aspirin, exhibits analgesic and anti-inflammatory properties. The di-aspirin derivatives have also been evaluated as potential anti-sickling agents. See Thompson et al., Res. Comm. in Chem. Path and Pharm., 48:381 (1985) and Walder et al., PNAS, 74:5499 (1977). In combination with glucose and sodium acetate or citrate, dibromoaspirin may significantly reduce gastrointestinal mucosal damage, as disclosed in U.S. Pat. No. 4,440.762 (Rainsford). 3,5-dibromoacetylsalicylic acid can be conveniently synthesized from dibromosalicylic acid and acetic anhydride in the presence of sodium acetate in water.
The synthesis of 3,5-dibromosalicylic acid has been described heretofore. U.S. Pat. No. 3,426,035 discloses a method of brominating salicylic acid utilizing epichlorohydrin or a lower alkylene oxide as an acid receptor in the nuclear bromination of aromatic compounds generally. A melting point of 221.degree.-223.5.degree. C. was reported. A second method in which the reaction is carried out in 50 percent paradioxane yields 3,5-dibromosalicylic acid with a reported melting point of 226.5.degree.-228.degree. C. (Neth. Applic. No. 6,610,859). In a third method described by Haksar et al., Vikram J., Vikram Univ., 6:67 (1962), salicylic acid is reacted with bromine in a medium of hot glacial acetic acid to yield a product with a melting point of 221.degree. C. All of these methods yield a product with melting points in the range of 221.degree.-228.degree. C., and all contain various impurities, principally monobrominated salicylic acid or decarboxylated brominated phenol.
Another disadvantage of prior art methods is the difficulty of removing the toxic solvents in the which bromination reaction is carried out. Although the bromination reaction can be more readily controlled in the presence of such solvents, these methods without further extensive purification steps are not useful for production of pharmaceutically acceptable material because of the risk of carry-over of toxic contaminants. In particular, methods utilizing paradioxane are unsuitable in pharmaceutical applications. Glacial acetic acid, utilized as a reaction medium by Haksar, is impractical in large-scale production because of the prohibitive expense of removing it. Finally, these methods are disadvantageous because the relatively high temperature conditions of bromination promote decarboxylation. In an industrial context, high temperature reflux procedures utilizing such flammable media also create a fire hazard and potential for escape of noxious fumes.
In addition to its usefulness as an intermediate in chemical syntheses, 3,5-dibromosalicylic acid has a number of direct applications where pharmaceutical purity is desirable. The compound is a member of a class of salicylates utilized singly or in combination as topical bactericides, fungicides, and treatment agents for various skin conditions. 3,5-dibromosalicylic acid is listed in the Merck Index (10th ed.) as a bactericide for this purpose. Direct topical compositions ideally should not contain impurities such as toxic solvents which may be absorbed through the skin. In other pharmaceutical applications the compound is used as a promoter of protein absorption in enteral administration (EP225189A2, Davis) and as an adjuvant in rectal drug delivery systems, as disclosed in U.S. Pat. Nos. 4,406,896 and 4,464,363 (Higuchi).